In recent years, intense study has been carried out on field effect transistors (FETs) using gallium nitride (GaN) material, for the use as a high-frequency power device or a power switching device. Nitride semiconductor material such as GaN can be used for making a various kinds of mixed crystal such as aluminum nitride and indium nitride. Therefore, as with conventional arsenic semiconductor material such as gallium arsenide (GaAs), nitride semiconductor material can be used for forming a heterojunction. An FET containing a heterojunction can operate at a high speed due to a two-dimensional electron gas with high mobility, which occurs at the heterojunction interface.
FIG. 12 shows a conventional FET using nitride semiconductor material. The semiconductor device 900 has an active section 990 and an inactive section 992. In the active section 990, finger electrodes serving as source electrodes 952, gate electrodes 956 and drain electrodes 954 are formed in a comb-like pattern. In the inactive section 992, a source pad 952a, a drain pad 954a and a gate pad 956a are formed.
FIG. 13 is a cross-sectional view of the semiconductor device 900. The semiconductor device 900 includes: a substrate 910 made of Si for example; a buffer layer 920 formed on the substrate 910; a channel layer 930 made of GaN and formed on the buffer layer 920; a barrier layer 940 made of AlGaN and formed on the channel layer 930, and protective films 950 and 960 formed on the barrier layer 940. Within the protective film 950, the source electrode 952, the gate electrode 956 and the drain electrode 954 are formed. Wiring patterns 962 and 964 are embedded in the protective film 960 so as to be located on the source electrode 952 and the drain electrode 954 respectively.
Commonly, a silicon nitride (SiN) film is used as the protective film 950. This is because silicon nitride prevents the formation of a surface state in the barrier layer 940.
The inactive section 992 formed around the active section 990 has ion implantation layers 932 and 942, which are formed by implanting ions of non-conductive impurities such as boron (B) and iron (Fe) and thereby eliminating channels. As the inactive section 992 is formed by ion implantation, no step occurs between the active section 990 and the inactive section 992. Therefore, it does not happen that the source electrode 952, the gate electrode 956 or the drain electrode 954 is disconnected by a step at the boundary between the active section 990 and the inactive section 992. Consequently, such a structure improves the yield during the process of forming the source electrode 952, the gate electrode 956 and the drain electrode 954.